Prior-art capacitive moisture and humidity sensors detect a change in capacitance due to a change in permittivity of a porous material layer due to cyclic absorption and desorption of water vapor. A disadvantage of these sensors is a slow response due to a time required for bulk material to reach equilibrium with water vapor in an external environment. Accordingly, the design of existing electrical hygrometers is constrained by a trade off between sensitivity and acceptable response time. Other disadvantages are a loss of calibration, reduction of sensitivity, and failure due to material absorption and surface retention of contaminants including fluids and particulates.
U.S. Pat. No. 6,222,376 B1 of Tenney, III and U.S. Pat. No. 4,429,343 of Freud disclose humidity sensors with interdigitated capacitor electrodes that are less sensitive to surface contamination. For these transducers, a majority of electric field coupling between the capacitor electrodes resides inside the material of the transducer. However, this approach further constrains sensor response time and does not provide a capability for reliable operation in an environment of fine airborne particulates.
Prior-art capacitance-based instruments used to measure the moisture content of agricultural products require a sample of fixed volume to be placed in a measurement cell. The permittivity or dielectric constant of the sample is then measured by a parallel-plate capacitor and a value of moisture content is determined from calibration data. The temperature dependency of permittivity is compensated for by measuring the temperature of the sample. Standard calibration curves of moisture content vs. dielectric constant at different temperatures exist for many agricultural products and industrial materials.
The density and packing fraction of grain and granular products strongly influence a measured value of permittivity. Small-volume samples are loosely packed and packing density can vary from sample-to-sample. This uncertainty is avoided in part by capacitive moisture instruments that simultaneously weight a measurement sample when its volume is known. These prior-art instruments are expensive and are not configured to provide in-situ measurements of commodities stored in bins, silos, and hoppers.
Humidity is the most important and difficult to control environmental parameter in greenhouses, particularly at high levels where the partial pressure of water vapor approaches saturation. Humidity affects the quality, yield, and health of plants. Greenhouse management is generally concerned with the control of relative humidity (RH) and vapor pressure deficit (VPD). RH is primarily used for disease control and VPD is used for transpiration control. Psychrometers with wet and dry elements are generally the only practical instruments available to measure high levels of humidity and to determine VPD in greenhouses and fog houses. They are too expensive and complex to be deployed in the numbers necessary for multi-zone climate control.
A critical concern in greenhouse management is to prevent the formation of freestanding water on plant surfaces and to prevent condensation from dripping on leaves. Accordingly, a need exits for networks of low-cost, chemically resistant condensation alarms.
A well known practice in the art of moisture and humidity measurement is to control the temperature of a transducer with a heating element or a thermoelectric heating and cooling module. This practice can enhance measurement accuracy and extend a range of measurement.